Over the last decade new research has offered the strong possibility that some aggressive behavior may be modulated by brain neurotransmittors. Recent research using human subjects has demonstrated a relationship between psychopathological syndromes, including excessive aggressive and impulsive tendencies and low 5-HIAA levels. The purpose of this study was to assess the relationship between serotonin metabolite (5-hydroxyindoleacetic acid, 5-HIAA) concentrations in cerebrospinal fluid (CSF) and behavior in two species which exhibit aggression at similar rates but distinctly different degrees of severity, sooty mangabeys and rhesus macaques. The two species were studied over a three month period in the Spring and a three month period in the Fall. The study showed significant inverse relationships between the total number of moderate and severe aggressive episodes initiated by some subjects and concentrations of 5-HIAA, however, there was no significant difference in 5-HI AA concentrations between species. Thus, while this study found no between species differences in 5-HIAA, results did strongly suggest an inverse relationship between escalated aggression and levels of 5-HIAA thus confirming previous reports of human subjects.